The overall goal of the proposed research is to determine the primary mechanosensor of mammalian cells for fluid flow. Various responses of different cell types to fluid shear stress have been explored on a biochemical basis, e.g. nitric oxide production or activation of G proteins. The hypothesis is that fluid shear stress increases membrane fluidity, thus activating heterotrimetric G proteins. Preliminary experiments using fluorescent membrane probes strongly support the hypothesis. The proposed project will focus on microscopic techniques that build on fluorescence and real-time computer image processing. Three alternative methods well be applied: Fluorescence recovery after photobleaching, fluorescence polarization anisotropy, and environment-specific fluorescent probes. All three methods allow the probing of fluidity properties on a sub- cellular scale, although the physical principles are fundamentally different. Therefore, the individual results will be used to mutually support and complete the overall findings. Furthermore, location specific effects, e.g. force accumulation at the cell-cell junction, can be investigated. If successful, the study will provide a fundamental understanding on how the endothelium senses hemodynamic forces as well as an insight into the shear-induced force distribution within the cell and on a macroscopic scale.